Glazing systems with silica aerogel for energy savings in buildings

Abstract

Innovative glazing systems with silica aerogel in interspace were investigated for energy saving in buildings. Aerogel is a promising transparent insulating material, due to its low thermal conductivity (down to 0.010 W m⁻¹ K⁻¹), high solar factor, high daylight transmittance and remarkable lightweight. Four samples were constructed with float and low-e glasses and granular or monolithic aerogel in interspace (14 mm thickness). The main optical characteristics of the samples were measured, in order to estimate the light transmittance τ_v, the solar factor g and the colour rendering index R_a, in compliance with EN 410/2011. Finally, the thermal transmittance was calculated. The monolithic aerogel glazings showed the best performance with respect to granular systems, both for light transmittance (0.62 between two 4 mm float glasses) and thermal insulation (0.6 W m⁻² K⁻¹). The solar factor was 0.74. Results showed a very promising behaviour of aerogel windows when compared to the windows normally used in Italy and in EU countries: monolithic aerogel between two 4 mm float glasses gave a 62% reduction in heat losses, with a 17% reduction in light transmittance when compared to a double glazing with a low-e layer; a high solar factor and colour rendering were also assured.

Introduction

The building sector has a relevant role in greenhouse gas emissions, where the energy consumption in residential and commercial buildings may enhance research and development of high performance windows. Transparent openings are very important for natural lighting, so they could be as large as possible for visual comfort and electric energy saving in illumination plants. But windows are also the weakest part of the thermal building envelope and they could be as small as possible in order to reduce energy consumption for heating and air conditioning. Advanced transparent superinsulation materials could satisfy both the requirements, due to their high thermal insulation and high light transmittance [1], [2].

In this context innovative materials such as silica aerogel are regarded as very interesting for use in highly energy-efficient windows [3].

Aerogels were discovered by Kistler 80 years ago [4] and they are dried gels often obtained by means of a very complex synthesis in supercritical drying conditions. The obtained structure (a cross-linked internal structure of silicon dioxide (SiO₂) chains and a large number of very small air-filled pores) is extremely light (density is in the 50–200 kg m⁻³ range) and has significant physical, thermal, optical and acoustic properties, depending on both the silica source and the preparation process [5], [6], [7]. Aerogel has in fact the lowest thermal conductivity among solid materials (down to 0.010 W m⁻¹ K⁻¹ at room temperature, depending on the pressure). Nevertheless the material is very fragile and the contact with water must be avoided: in commercial applications, aerogel may be used in vacuum conditions, with evident advantages in terms of thermal insulation. Finally, relating to safety aspects, the material is not carcinogenic, non-flammable and non-reactive [8].

A wide range of applications of aerogels are reported in the literature [9], but at the moment the building applications as thermal and acoustic insulation material are considered. Aerogels have interesting optical properties, such as high light and solar transmittance, and excellent thermal insulation properties, if compared to glass. They are used as transparent walls in solar collectors and in office buildings [10], [11], [12].

In this case opaque and transparent aerogels were developed [13]: the opaque ones are flexible blankets obtained by adding fibres in the gel before the drying process, in order to reduce the fragility (thermal conductivity about 0.13 W m⁻¹ K⁻¹), but the cost is about 10 times higher than a conventional material with similar performance; the transparent ones are available as panes or granules: glazing systems with monolithic aerogel are not yet used in mass production [14], [15], but during the year 2005 many types of translucent granular aerogel appeared on the market for daylighting systems [16], [17]. The different systems (polycarbonate panels, structural panels, insulated glasses) offer excellent thermal performance, a good solar heat gain and a good sound insulation [18], [19].

Aerogel, especially the monolithic pane, appears as a very transparent and lightweight material (Fig. 1), but it has a tendency to scatter the transmitted light, resulting in a hazy picture when objects are viewed through it. Furthermore, the colour rendering seems also to be modified and this is a problem in using aerogel in clear windows. In fact glazings can greatly affect the characteristics of the incoming light; they can also modify the colour appearance of surfaces and objects, due to the alteration of the solar radiation spectral distribution. This behaviour contributes significantly to the comfort and visual satisfaction of human beings in indoor environments [20]. Aerogel seems to have lower light transmission performance than other transparent materials; but due to its very good thermal properties, it could be interesting to investigate also this characteristic when assembled with conventional glasses.

In the present paper the optical and thermal properties of advanced glazing systems with aerogel were investigated, in order to evaluate their possible employing in buildings for energy savings.